The overall objectives of this renewal application are to delineate and characterize specific surface proteins of the mollicute Mycoplasma fermentans that are associated with pathogenic interactions with the host, and to understand underlying genetic features of this small prokaryotic pathogen that may provide adaptive diversity and variation. Implicated as a causal agent of a fatal acute respiratory distress syndrome, and an uncommonly prevalent agent isolated from patients with AIDS, M. fermentans plays a potentially important role in human disease either as a primary or contributing pathogen. Factors modulating several host cell functions have been associated with M. fermentans. Surface membrane lipoproteins displaying multiple forms of variation in surface structure and expression have been documented and a large(>25 kb), repetitive chromosomal element (REP) that contains multiple contiguous ORFs encoding expressed lipoproteins and other putative virulence factors has been recently identified. Efforts will be focused on characterizing this unique genetic element and its likely role in genetic variation and surface diversity. Lipoproteins are also potent immunomodulatory molecules. Recently, a short peptide sequence of a mitogenic lipopeptide purified from M. fermentans has been determined. It is intended to analyze the gene structure corresponding to this lipopeptide. Using standard molecular biology and immunobiochemical procedures, the proposed project will (i) characterize the primary gene product of the bioactive lipopeptide, (ii) characterize clones containing complete REPs, (iii) compare the chromosomal distribution of REPs among the strains, and (iv) identify variants of a REP-encoded surface lipoprotein. Overall, these studies should provide new knowledge on bioactive lipid compounds and genetic variation, both potentially crucial for the adaptation and pathogenicity of this organism.